1. Field of the Invention
This invention relates to a novel rotary optical head apparatus, and particularly to a rotary optical head apparatus for use with an optical recording and reproducing apparatus of the tape type wherein a beam spot of a laser beam scans an optical record medium in the form of a tape while the tape is advanced to effect recording and/or reproduction of a signal. More specifically, the present invention relates to a rotary optical head apparatus which includes a rotary member carrying thereon an objective lens adapted to focus a laser beam on an optical record medium in the form of a tape such, for example, as a magneto-optical tape, and another rotary member carrying thereon an image rotator interposed in a beam path of a laser beam to the objective lens, that is, a prism for maintaining a corresponding relationship between the direction of deflection of a laser beam and the direction of movement of a beam spot. The present invention provides a novel rotary optical head apparatus wherein a support structure for two rotary members such as those described above is improved so that axes of rotation of the two rotary members can coincide with each other with a high degree of accuracy and the linearity of a scanning line of a beam spot to be scanned on an optical record medium in the form of a tape can be ensured with a high degree of accuracy.
2. Description of the Prior Art
An optical recording and reproducing system is already known wherein information in the form of a signal is written onto an optical record medium and such recorded information is reproduced using the energy of a laser beam.
Recording and reproducing systems of the type mentioned have been partly put into practical use, but they use a record medium of the disk type: that is, an optical disk. On the other hand, several attempts have also been made to use a record medium of the tape type.
FIG. 9 shows a typical conventional rotary optical head apparatus employed in recording and reproducing apparatus in which an optical record medium in the form of a tape is used. The rotary optical head apparatus shown is generally denoted at a and includes a tape winding drug b formed from a transparent material and having a generally cylindrical profile. When an optical record medium in the form of a tape, that is, an optical record tape c, is to be recorded or reproduced, it is wound substantially spirally over a predetermined winding angle around an outer periphery of the tape winding drum b and is fed by the tape winding drum b. The rotary optical head apparatus a further includes a first rotary member d in the form of a thick disk mounted on the inside of the tape winding drum b and connected to a motor e by which it can be rotated. The first rotary member d has a light-passing hole f formed radially therein. The hole f opens on the radially inner side thereof to a central portion of a lower face of the first rotary member d and on the radially outer end thereof to an outer periphery of the first rotary member d. A lens holder g carrying therein an objective lens set (not shown) is disposed adjacent to the radially outer end of the light-passing hole f. A mirror h mounted in the light-passing hole f is inclined at an angle of 45 with respect to the axis of the light-passing hole f. Thus, a laser beam i introduced from below and impinging on the mirror h is focused by the objective lens in the lens holder g so that it forms a beam spot j of a predetermined diameter on a record face of the optical record tape c wound around the transparent tape winding drum b.
The rotary optical head apparatus a further includes an optical block k which includes a laser light source, a collimator lens, a beam splitter, an optical detector and so forth (not shown). The rotary optical head apparatus a further includes a galvanomirror for the tracking servo, a second rotary member m carrying an image rotator n thereon and connected to be rotated by a motor (not shown), and a focusing block o including a focusing lens and so forth (not shown). The galvanomirror p, second rotary member m and focusing block o are arranged on the axis of rotation of the first rotary member d.
A laser beam i radiated from the optical block k impinges on and is reflected by the galvanomirror p. The galvanomirror p changes the direction of propagation of the laser beam i by a substantially right angle. The laser beam i then successively passes through the image rotator n and focusing block o and impinges on the reflecting mirror h so that it is focused upon the record face of the optical record tape c as described above. Thus, when the optical record tape c is advanced and the first rotary member d is rotated, the beam spot j of the laser beam i scans the record face of the optical record tape c in a direction oblique to the direction of advance of the optical record tape c. Consequently, recording of a signal is effected while forming a helical record track or reading of a recorded signal is effected while the beam spot j of the laser beam i scans a previously-formed record track.
When the galvanomirror p is rocked about a certain pivot axis (not shown), the beam spot j moves on the optical record tape c in a direction substantially perpendicular to the direction in which the record track extends. However, while the galvanomirror p is mounted so as not to rotate about the axis of the first rotary member d, the reflecting mirror h normally rotates about that axis. Consequently, the direction of deflection of the laser beam i caused by the rocking motion of the galvanomirror p does not coincide with the direction of movement of the beam spot j. The image rotator n is a prism for correcting such non-coincidence, and the second rotary member m carrying the image rotator n thereon is rotated at an angular velocity equal to one-half the angular velocity of the first rotary member d. Consequently, coincidence is normally kept between the direction of deflection of the laser beam i by the galvano-mirror p and the direction of movement of the beam spot j.
Rotary optical head apparatus as described above is disclosed, for example, in Japanese laid-open patent publication No. 112234/1987.
In such rotary optical head apparatus a, it is very important that the axis of rotation of the first rotary member d and the axis of rotation of the second rotary member m be aligned with each other with a high degree of accuracy.
In particular, if the axes of rotation are not in register with each other or are inclined relative to each other, in other words, if they are not colinear, then the position at which the laser beam i reflected by the galvanomirror p impinges on the reflecting mirror h varies cyclically, and consequently the beam spot j oscillates cyclically in both the direction of the advance and the widthwise direction of the optical record tape c. Consequently, the linearity of the scanning line of the beam spot j deteriorates significantly, which causes a significant tracking error.